An A/D converter of an successive approximation type is a kind of an A/D converter for quantizing an analog signal. The successive approximation type A/D converter successively (per bit) compares an analog potential of an inputted analog signal to a variable reference voltage, and successively changes the reference voltage so that the reference voltage approximates to a voltage level of the inputted analog signal in accordance with a result of the comparison successively obtained to thereby obtain the digital signal as an output result (for example, see No. H07-193503 of the Publication of the Unexamined Japanese Patent Applications (Page 3-4 and FIGS. 1-2).
FIG. 11 is a circuit diagram illustrating a configuration of a conventional successive approximation type A/D converter. Referring to reference numerals shown in the drawing, 10 denotes an analog input terminal, 21 denotes a comparator of a chopper type, 22 denotes an analog switch for short-circuiting input and output terminals of the comparator 21, 23 denotes a control inverter for controlling ON/OFF of the analog switch 22, 24 denotes an inverter for buffering in a first stage, 25 denotes an inverter in a second stage, 70 denotes a sample hold circuit, 71 denotes a capacitance array circuit, 72 denotes a reference voltage generating circuit, 73 denotes a ladder resistance circuit, 74 denotes a high-potential side reference power supply VDD, 75 denotes a low-potential side reference power supply VSS, 76 denotes a group of analog switches, 80 denotes a control circuit, and 90 denotes a latch circuit.
In a sampling period, a control signal Sc from the control circuit 80 is set to “H” level so that the analog switch 22 is turned on. Then, the input and output terminals of the comparator 21 are short-circuited, and a voltage value of half a full-scale voltage which is able to be converted by A/D converter (½ VDD) is generated. In response to the voltage, the capacitance array circuit 71 charges all of capacitances thereof using a potential difference between the ½ VDD and a voltage level of an analog signal inputted from the analog input terminal 10 and holds as an electric charge. Next, in a successive comparison period, the analog switch 22 is turned off, and the comparator 21 is operated as a comparator for inputting a voltage from the capacitance array circuit 71 so that a value of an input analog signal held in the capacitance array circuit 71 is compared to an output voltage level of the reference voltage generating circuit 72. When the voltage level of the input analog signal is higher than the output voltage level, an output value of the comparator 21 is buffered in two inverters 24 and 25, and a reference voltage value is held at a high potential with a first bit (MSB) of a digital output being set to “1”. When the voltage level of the input analog signal is lower than the reference voltage, and the first bit of the digital output is set to “0” to return the reference voltage value to zero again. The digital output of the comparator 21 is retained as a digital value in the latch circuit 90.
When the foregoing operation is repeated until a value of a n'th bit is determined, a quantized data of the input analog signal is obtained as the digital output of n bits.
Further, the capacitance array circuit 71 is used, that holds the inputted analog values as the electric charge, and weighting corresponding to a context of the respective bits is adjusted based on the output voltage level of the reference voltage generating circuit 72 and capacitance value. In the bits where the weighting is adjusted based on the output voltage level of the reference voltage generating circuit 72, a voltage ½ VDD, ¼ VDD, ⅛ VDD, . . . , or 1/N VDD (N is a bit number weighted by the reference power supply) corresponding to approximately ½, ¼, ⅛, . . . or 1/N of the full-scale value, or 0, is added to the output voltage level of the reference voltage generating circuit 72 in the relevant bit for comparison. As a result of the comparison, when the voltage level of the input analog signal is higher than the reference voltage, the digital output is set to “1”. When the voltage level of the input analog signal is lower than the reference voltage, the digital output is set to “0”, and the reference voltage value is returned to zero again.
When the group of analog switches 76 are controlled by a timing signal Ss from the control circuit 80 and a voltage voltage-divided by the ladder resistance circuit 73 is connected to a capacitance of the relevant bit in the capacitance array circuit 71, the reference voltage can be changed.
There is a problem in the sampling period, however, that is, the ½ VDD, which is an intermediate voltage, generated in the comparator 21 of the chopper type is loaded to a gate of the buffering inverter 24, and current consumption is excessively increased due to a through current flowing in the inverter 24 and the subsequent circuits.
Further, a current flow is constantly supplied to the ladder resistance circuit 73 in the reference voltage generating circuit 72, which also increases the current consumption.
In the case of providing an A/D converter in an LSI such as a microcomputer, the A/D converter is often operated by clocks of a plurality of frequencies, in this case a comparator with a high performance is used to realize a normal operation at a highest operation speed. However, when the A/D converter is operated by a clock of a low frequency, the performance of the comparator becomes excessive, which largely increases the current consumption.